Recently, planar display devices such as liquid crystal display devices and organic electroluminescence devices are highlighted as the electronic display device. Since the liquid crystal display device has a smaller size and is more compact than the cathode ray tube display device, various appliances equipped with the liquid crystal display device are being widely used. For example, the demand for downsizing various appliances such as personal computers, video cameras and other consumer appliances is great in the market. Specifically, portable appliances having a small size such as lap top computers and cameras having a liquid crystal monitor have been widely used. It is essential in these appliances that the appliances are equipped with a liquid crystal display device. The demand for improved performance of displays such as display with colors and improved luminance is also great. Since the liquid crystal display devices do not emit light spontaneously, a light source such as a back light and a front light is necessary.
In the liquid crystal device, the linearly polarized incident light is modulated based on the electro-optical property of the liquid crystal layer and visualized by the polarizing plate at the exit side as a signal expressed by variation in the transmission and the color. Since the polarization of light is used as the principle of display, the polarizing plate is the essential member. The polarizing plate is a device which changes natural light into a linearly polarized light. At present, polarizing plates obtained by laminating an optically transparent protective film having a sufficient mechanical strength to one or both faces of a polarizing film, which is prepared by dying a substrate film made of a polyvinyl alcohol film with a dichroic material such as iodine and dichroic dye or by absorption of the dichroic material into the substrate film, followed by stretching and orientation of the resultant film, are mainly used as the polarizing plate for the liquid crystal display devices. As the protective film, in general, a triacetylcellulose film is used. In the liquid crystal display device, in general, the polarizing plate is disposed at both of the exit side and the incident side of the liquid crystal layer.
When a display device such as the liquid crystal display device is used, light from the outside is occasionally reflected at the surface of the display, and difficulty arises in watching images on the display. Overcoming this problem is becoming more important.
Various treatments for preventing reflection of light and various anti-glare treatments have been made on various display devices to overcome the above problem. As one of such treatments, antireflection films are used for various display devices.
As for the polarizing plate for the liquid crystal display device described above, it is attempted that an antireflection layer is disposed on the protective film of the polarizing plate to prevent the problem due to the light from the outside (for example, Patent Reference 1). It is further required for the protective film for the polarizing plate that durability to impact probable in the ordinary condition and to substances present in the ordinary environment be exhibited since the protective film is placed at the outermost surface of the visual side.
The formation of the antireflection layer is, in general, conducted by alternately laminating high refractivity layers and low refractivity layers in accordance with the physical vapor deposition (PVD) process or the chemical vapor deposition (CVD) process. It is also conducted that a hard coat layer or an antifouling layer is formed to provide the durability and the wear resistance described above to the protective film for the polarizing plate.
Patent Reference 1
Japanese Patent Application Laid-Open No. 2002-286932
However, it was found by the studies by the present inventors that, when a polarizing plate was prepared by using a conventional protective film for polarizing plates and mounted to a liquid crystal display device, problems arose in that the distribution of luminance and the distribution of color difference became uneven, and images showed fluctuation, flicker and uneven color.
As the result of the study on the cause of the problems, it was found that, since triacetylcellulose exhibiting a great absorption of water was used in conventional protective film for polarized plates, the easiness of the vapor deposition tended to be different at portions of the film. As the result, the thickness of the antireflection layer became uneven within the surface of the film, and the reflectance fluctuated.
Before the antireflection layer is vapor deposited, it is necessary that the roll of the raw material film be wound and unwound several tens of times to remove water adsorbed to the roll of the raw material film. It was found that microscopic damages were formed on the surface of the film during the repeated winding and unwinding, and wrinkles were formed in the film due to the change in the temperature during the repeated winding and unwinding. These phenomena caused the fluctuation in the reflectance.
Under the above circumstances, the present invention has an object of providing a protective film for polarizing plates which has a uniform spectral reflectance within the surface of the film and is provided with the function of preventing reflection which can suppress fluctuation in the distribution of luminance and the distribution of color difference when the film is disposed on the surface of the visual side of a display device such as a liquid crystal display device.